This was a great read, and I am glad to see someone else out there who not only knows about Ulmus thomasii but also recognizes its amazing bonsai potential. It is certainly a lesser-known native species, and to me, that makes it even cooler and more appealing to work with. I work for a large nursery here in S.E. Wisconsin, and we finally have some good crops available of seed-grown rock elms sourced from a couple of small local populations. We sell them in 5-gallon containers and hope to sell them to both homeowners and restorationists to increase local populations.
I currently have two in training, and they are off to a great start. But it will be quite a while before they are nice bonsai. I've attached a photo of my favorite of the two I have in training. It's currently growing in a 5-gallon rootmaker pot.